Volcanoes in the different regions of the Earth. 239 



from the phenomena of volcanoes. La Place has even attempted 

 to determine the depth at which the earth may be considered as 

 a melted mass. Whatever doubts may be entertained, notwith- 

 standing the respect due to so great a name, as to the numerical 

 accuracy of such a calculation, it is not the less probable, that 

 all volcanic phenomena arise from a single cause, which is the 

 communication, constant or interrupted, that exists between the 

 interior of our planet and the external atmosphere. Elastic va- 

 pours, by their pressure, raise through deep crevices the sub- 

 stances which are in a state of fusion, and which are oxidized. 

 Volcanoes are, so to speak, intermittent springs of earthy mat- 

 ters. The fluid mixtures of metals, alkalies and earths, which 

 condense into currents of lava, flow gently and slowly, when, 

 on being raised up, they once find an issue. It was in this man- 

 ner that, according to Plato's Phcsdos, the ancients represented 

 all the torrents of fire as emanations of the Pyriphlegeton. 



To these considerations may I be permitted to add another of 

 a bolder character. It is perhaps in the internal heat of the 

 earth, a heat which is indicated by experiments made with the 

 thermometer, and the phenomena of volcanoes, that the cause of 

 one of the most astonishing phenomena which the knowledge of 

 petrifactions presents to us resides. Tropical forms of animals, 

 arborescent ferns, palms and bamboos, occur imbedded in the 

 frozen regions of the north. The primitive world every where 

 discloses to us a distribution of organic forms, which is.in oppo- 

 sition to the presently existing state of climates. To solve so im- 

 portant a problem, recourse has been had to a great number of 

 hypotheses, such as the approach of a comet, the change of ob- 

 liquity of the ecliptic, the increase of intensity of the solar heat. 

 None of these hypotheses has been able to satisfy at the same 

 time the astronomer, the natural philosopher and the geologist. 

 As to my own opinion on the subject, I leave the earth's axis in 

 its position, I admit no change in the radiation of the solar disk, 

 a change by which a celebrated astronomer thought he could ex- 

 plain the good and bad harvests of our fields; but I ima- 

 gine that in each planet, independently of its relations to a cen- 

 tral body, and independently of its astronomical position, there 

 exist numerous causes of developement of heat, whether by the 

 chemical processes of oxidation, or by the precipitation and 

 changes of capacity of bodies, or by the augmentation of the 



